1. Field of the Invention
The present invention relates to a process for producing graphite materials which are well suited to a negative electrode material for use in a lithium secondary battery. More particularly, the present invention pertains to a process for producing graphite materials for a negative electrode used in a lithium secondary battery which process is capable of uniformizing the variation in the physical properties of graphite materials caused in a graphitizing treatment; stabilizing the resultant graphite quality; and also increasing its productivity. The present invention is also concerned with a process for producing graphite materials which enhances the qualitative stability thereof at the time of employing two or more kinds of mixed graphite materials.
2. Description of the Related Arts
In recent years, electronic appliances have achieved rapid technological development searching for miniaturization, lightweight property and high performance. Thereby, portable electronic appliances have remarkably been widespread, which are typified by celler telephone, PHS (Personal Handy-phone System), cam coders and personal computers. Accompanying the development and progress of these new electronic appliances, new secondary batteries have appeared on the market, including a nickel-hydrogen battery and a lithium secondary battery.
In particular, a lithium secondary battery is imparted with various advantages such as high energy density, high electromotive force, a wide range of operating termperatures due to the employment of a nonaqueous electrolyte solution, excellence in long-term preservation, lightweight property, miniaturization, and the like. Accordingly, the aforesaid lithium secondary battery is expected to find practical use of high performance batteries for use in electric automobiles, electricity storage and the like.
The lithium secondary battery has realized improvement in its performance and safety by using, in a negative electrode, a carbon material in place of metallic lithium. In more detail, in the case where a carbon material is used in a negative electrode, lithium ions are incorporated in the carbon structure, whereby lithium dendrite is prevented from forming, and thus the safety of the battery is drastically enhanced. There is proposed the use of pitch-based milled graphite fibers as a negative electrode for a lithium secondary battery, for example, in Japanese Patent Application Laid-open Nos. 325967/1993 (Hei-5), 36802/1994 (Hei-6), and 90725/1995 (Hei-7).
The milled graphite fibers used as a negative electrode for a lithium secondary battery are obtained by, for example, spinning a pitch as a starting material, infusibilizaing the spun fibers, then pulverizing (milling) the resulting carbonized materials, regulating the particle size thereof, and thereafter graphitizing the regulated particles of the carbon materials.
The graphitization treatment is put into practice usually by packing carbon materials which are obtained by carbonization followed by milling, in prescribed graphite-made boxes, placing the boxes in a graphitization furnace, and heating the boxes at a high temperature of 2000.degree. C. or higher.
In order to enhance the graphitization efficiency, though depending upon the size of the graphitization furnace, the graphitization is simultaneously carried out in such a manner that a plurality of boxes each accomodating a carbon material are arranged in a plurality of rows in a plurality of stages.
The temperature in the graphitization furnace, although depending upon the heating system and the extent of thermal insulation, usually becomes higher as the position of the box comes close to the central portion of the furnace. Such difference in temperature tends to make difference in the degree of graphitization after the graphitization treatment depending upon the position of each box arranged in the furnace, that is, the closer the position of the box to the center of the furnace, the higher the degree of graphitization, on the contrary, the closer the position of the box to the wall of the furnace, the lower the degree of graphitization. In addition, there is a problem that the particle size distribution of the graphite is made non-uniform even in the same box because of charging operation or bubbling and the like due to the gases generated through graphitization.
The above-mentioned problems are primary contributors to the fact that the same performance is not necessarily realized in the batteries produced from graphite materials which are simultaneously treated in the same graphitization furnace, but which are located in different boxes or which are located in differrent places of the same box.
On the other hand, for the purpose of complementing the items to be improved in a single kind of material, researches have been made on the mixed use of various materials.
With regard to the embodiment of the mixing, the scope of the researches covers wide fields including a variety of combinations of the materials to be mixed, even if limited thereto, such as graphite base/graphite base, carbon base/carbon base, graphite base/carbon base, graphite base/carbon base that are incorporated with an electroconductive material and a mixed system of at least three kinds of materials.
In regard to the mixing of graphite base/graphite base, for example, there are disclosed the mixing of spheroidal graphitized carbon particles/graphitized carbon short fiber in Japanese Patent Application Laid-Open No. 111818/1994 (Hei-6); the mixing of graphite materials comprising spheroidal particles (MCMB)/graphite fine particles having a smaller particle diameter than MCMB (natural graphite, artificial graphite or the like) in Japanese Patent Application Laid-Open No. 37618/1995 (Hei-7); the mixing of graphite materials comprising spheroidal particles (MCMB) or the like/fibrous graphite in Japanese Patent Application Laid-Open No. 287952/1996 (Hei-8); the mixing of milled mesophas pitch base carbon fibers/natural or artificial graphite in Japanese Patent Application Laid-Open No. 83608/1996 (Hei-8); and the mixing of graphite material/graphite material different in bulk density from each other in Japanese Patent Application Laid-Open No. 180873/1996 (Hei-8).